The present invention relates to a fault diagnosis device for an exhaust gas recycle (which is hereinafter referred to as EGR) control unit which controls the return of an exhaust gas of an engine to an intake pipe.
As an conventional fault diagnosis devices for EGR control units, there is known a fault diagnosis device which is disclosed in Japanese Patent Laid-Open No. Sho.62-51746. This conventional fault diagnosis device detects the operating states of an engine when a return valve for opening and closing an exhaust gas return pipe which flows the exhaust gas back to an intake pipe is opened and closed. Thus the device stores the detected values of the operating states separately, compares a difference between the two detected values with a predetermined range, and, when the difference is found within the predetermined range, sets off an alarm that the EGR control unit is out of order.
Due to the fact that the above-mentioned conventional fault diagnosis device for an EGR control unit is arranged in the above-mentioned manner, when any trouble is detected in the EGR control unit, the flow rate of the EGR is changed suddenly when the return valve for the return pipe is closed from the open state thereof or is opened from the closed state thereof, as the result, the torque of the engine produced is changed suddenly. This gives a driver an uncomfortable feeling.